Further studies on the mechanism of immunosuppression of delayed-type hypersensitivity using the guinea pig-sheep erythrocyte system will be conducted. The results obtained from the aforementioned study will be applied to investigations pertaining to the possible interrelationships which exist between delayed-type hypersensitivity, lymphocytic factors associated with the latter, and acquired cellular resistance to facultative intracellular parasites. Attempts will also be made to design an appropriate in vitro model with which to study delayed-type hypersensitivity and acquired cellular resistance and with which the significance of these immunological phenomena in relationship to tumor immunity can be investigated.